1. Field of the Invention
The invention relates to electrical switching apparatus, such as, for example, circuit breakers and, more particularly, to molded case circuit breakers.
2. Background Information
Circuit breakers, such as molded case circuit breakers, include at least one pair of separable contacts. A first contact is fixed within the molded case and a second movable contact is coupled to an operating mechanism. These separable contacts are in electrical communication with either the line or the load coupled to the circuit breaker. The operating mechanism moves the movable contact between a first, open position wherein the movable contact is spaced from the fixed contact, and a second, closed position wherein the fixed and movable contacts are in contact and electrical communication. The operating mechanism may be operated manually or by a trip mechanism.
Circuit breaker molded cases are typically constructed of a glass polyester or other molded material. Generally, these molded cases are a two-piece molding.
When interruption of relatively high electrical currents occurs, hot gases are formed that can exert significant pressure on the molded case. The outer walls of the molded case may be subjected to relatively high pressure and, thus, severe stress, resulting from the extreme expansion of gasses. Because the gas pressure is relatively high and the molded case walls are relatively thin, the molded case is susceptible to breaking due to the pressure and the resulting stresses. The exterior walls of molded case circuit breakers have, thus, typically been a weak link for case strength and a limiting factor in increasing the interrupting ratings of circuit breakers.
Typically, the molded cover and molded base of molded case circuit breakers are joined by conventional thread cutting screws, which cut threads into the molded material. However, that molded material may crack apart and the conventional thread cutting screws may be pulled out of the molded case during the process of interruption.
One prior proposal mounts the internal components of the circuit breaker poles on or in metal, which then attaches to the molded case.
U.S. Pat. No. 4,595,896 discloses a circuit breaker in which each pole of the circuit breaker has a reinforced molded housing. The walls of the housing are provided with reinforcing molded ribs to withstand the high pressures caused by gases generated by the arc during interruption.
U.S. Pat. No. 6,452,470 discloses that outward gas forces have a tendency to put downward pressure on portions of side walls that connect with the bottom of the molded base. Support members provide underneath support for the side walls, thereby substantially preventing bottom corner areas of the molded case from being unduly stressed and bent by the aforementioned forces.
U.S. Pat. No. 6,970,059 discloses a circuit breaker in which the rigid legs of a slot motor reinforce the exterior walls of the molded housing.
It is known in some relatively large molded case circuit breakers to use only machine screws and hex nuts for the purpose of holding the molded cover in place on the molded base. The same types of machine screws and hex nuts are employed in both the molded case external walls and the molded case internal walls.
It is also known in other relatively large molded case circuit breakers to use machine screws and hex nuts within relatively thick internal walls for the purpose of holding the molded cover in place on the molded base, and to use thread cutting screws within relatively thinner external walls for the same purpose of holding the molded cover in place on the molded base.
There is room for improvement in electrical switching apparatus employing molded enclosures.
There is also room for improvement in molded case circuit breakers.